Lake Suwa’s Shinto Legend and the Oldest Lake Ice Record on Earth: What It Tells Us About Climate Change and Variability

April 26, 2016

Lake Suwa’s ice ridge is the subject of Shinto Legend in Japan and of great interest to lake and climate scientists. (Photo credit: Masahiro Miyasaka)

By Lisa Borre

Shinto priests observing an ancient legend recorded ice freeze dates on Lake Suwa in Japan starting in the 15th Century. On the other side of the world, a local merchant began a tradition of recording ice thaw events on the Torne River in Finland in the 17th century. Both traditions continue to the present day and represent the oldest known records of lake freeze and river thaw observations on Earth. Thanks to the careful preservation and documentation of these records and the perseverance of a team of modern-day scientists, the records have been analyzed to find out what they can tell us about climate change and variability.

Lake Suwa is a shallow lake nestled in the Kiso Mountains of Nagano Prefecture. According to Shinto legend, the male god Takeminakata would cross the lake to visit the female god Yasakatome at her shrine on the other side. The crossing was evidenced by the god’s footsteps on the ice that left a sinusoidal ice ridge, known as the omiwatari in Japanese.

Like the 15 generations of priests before him who were observing the ancient legend, Kiyoshi Miyasaka, the present priest at the Yatsurugi Jinja Shrine and Tenaga Jinja Shrine, diligently records the omiwatari event, although it is happening much less frequently in recent years, according to a study published today in Nature Scientific Reports.

Kiyoshi Miyasaka, a Shinto priest, pointing out some of the records on lake ice and the omiwatari. His data sheet summarizing the records is on the table. (Photo credit: J.J. Magnuson)

The formation of ice ridges is a well-known phenomenon on northern temperate lakes. The scientific explanation is that ice ridges form with variations of temperature. Cold nights and warm, sunny days cause the ice to expand and contract, forming cracks. Sometimes when the ice cracks, it is accompanied by a loud booming sound, known as “ice thunder.” Broken pieces of the relatively thin ice are forced upwards, creating a ridge.

Since regular record keeping began in 1443, the Shinto priests have recorded the day of complete ice cover, making it a very reliable dataset.

The Torne River flows southward from the Arctic to the Baltic Sea. A local merchant named Olof Ahlom started keeping ice breakup records in 1693, where the river flows past the small town of Tornio, Finland. There is a short gap in the ice record during the Russian occupation (1715-1721), but it is otherwise well preserved. The tradition continues with ice breakup guessing competitions and competitors vying for hour and minute accuracy, according to the study.

While different from the lake freeze event in Japan, the ice observations in Finland serve as a reliable record of river breakup dates.

Professor Emeritus John Magnuson at the University of Wisconsin Center for Limnology first began working with the Japanese and Finnish ice records in the 1990s as part of a research project to look at historical trends in lake and river ice funded by the National Science Foundation and published in 2000. This earlier work compared ice records back to the 1850s, but he remained interested in working with the Suwa and Torne datasets because they pre-dated the Industrial Revolution and because ice records based on direct human observation are so rare.

The Lake Suwa records presented an added challenge because of the changing Japanese calendar during the 18th and 19th centuries. During this period, the calendar changes were not consistent and very localized. In the end, the researchers could not reconcile the record, and these observations were removed from the analysis. Although Magnuson acknowledges his disappointment, he is quick to point out that the missing data does not detract from the main findings of the research.

One of the important findings of their research relates to trends in ice seasonality. Both locations exhibited more rapid rates of change consistent with warming – later ice freeze and earlier ice breakup – following the start of the Industrial Revolution in the 1840s, the study says.

In the last decade, Lake Suwa did not freeze five times, compared to three times in more than three centuries in the period before the 1800s, according to the study. Similarly, the Torne River has experienced five extreme warm years in the past decade, compared with four extreme warm years between 1693 and 1799. This is consistent with other studies showing that the likelihood of extreme events have increased for later ice freeze, earlier ice breakup and shorter ice duration for lakes across the Northern Hemisphere after the mid 1800s.

The research team’s findings were consistent with other studies that have found significant changes in the cycles of events such as the North Atlantic Oscillation and El Niño Southern Oscillation. These and others changes are contributing to less ice on lakes and rivers in a warming climate. Sharma reflected on the bigger picture implications of their research findings. “I was surprised to discover that we humans have been able to change these long term climate drivers with our actions,” she said.

Magnuson was surprised by one of the study findings that he thinks is significant: the Suwa and Torne data provide strong evidence that climate — the temperature as reflected in the ice dates — is not becoming more variable among years. “There is a definite warming trend, but the variability around the annual mean is the same as or less than it used to be,” he said. One caveat is that in the most recent years, the absence of ice cover shows up as an increase in variability.

It turns out that local factors, such as changes in land use, human population growth and even the installation of a floodgate and development of hot springs on Lake Suwa, had little influence on the patterns of change in ice freezing dates for the lake or ice break-up dates for the Torne River. Among other relevant findings, this helps put climate change factors into context for lake conservation and management efforts by confirming that global forces are also affecting the changes being observed on lakes and rivers across the globe.

What are the environmental implications for a world with less lake ice?

Research on lake systems as large as the North American Great Lakes and as small as seepage lakes in northern Wisconsin has shown that warming trends and less ice cover can affect everything from evaporation rates and water levels to the cycling of nutrients and biological processes in lakes. Warmer water also creates favorable conditions for algal blooms, including toxin-producing cyanobacteria blooms, thereby affecting water quality.

“For those of us who live in cold climates, ice is part of our sense of place,” Sharma said. “Ice is part of our cultural identity associated with winter. Ice influences our recreational activities, from ice fishing to ice skating and much more. With a changing climate, we need to better manage water resources or the situation in water-stressed regions, and those experiencing water quality problems, will be much trickier.”

The First Citizen Scientists?

With citizen science a trendy topic these days and a focus of some of my own work, I couldn’t help but ask Magnuson whether Shinto priests who made the omiwatari observations at Lake Suwa might also be considered some of the earliest citizen scientists.

“What is so interesting to me is that we were able to include in our analysis the longest ice records in the world based on direct human observation,” he said. This is something that distinguishes the study from other paleoclimate and modeling studies. “To have such long records from two very different freshwater systems in different parts of the world show the same general pattern is one of our key findings,” he said. “Our findings are entirely consistent with other research that shows a pattern of change after the Industrial Revolution.”

“Although the observations weren’t made in connection with any scientific study, the data were recorded and preserved in a way that provides extremely useful information for modern-day climate science,” Magnuson said.

An old photograph on the wall of the Yatsurugi Jinja Shrine. On Lake Suwa, the formation of omiwatari is followed by a purification process by the priests at the shrine and a celebration. The event and ice freeze dates have been recorded by 15 generations of Shinto priests, including the priest in the photo, who is the father of Kiyoshi Miyasaka, the present priest at Yatsurugi Jinja Shrine and Tenaga Jinja Shrine. (Image courtesy of J.J. Magnuson)

Magnuson visited Lake Suwa in 2005 to see the historical ice records for himself. Working with his Japanese collaborators and through translators over the years, he was impressed by the level of preservation of centuries-old observations written on rice paper. He was most impressed with Mr. Miyasaka, the present Shinto priest and keeper of the shrine’s omiwatari records, who was surprised to learn that they held scientific interest.

At the time of Manguson’s visit, Miyasaka expressed concern about the loss of ice cover on Lake Suwa. Recently he provided the research team with Suwa ice data for the period 2000-2014 in hopes of gaining insights from scientists about changes in this important local phenomenon.

Aono wrote in an email that the omiwatari didn’t appear on Lake Suwa in the 2015-2016 winter season. “It was not observed in 2014 or 2015 either. The last omiwatari was observed on Jan 22, 2013,” he added.

The study leaves no doubt that loss of ice cover on lakes and rivers in the Northern Hemisphere can be attributed to human-induced climate change. If current trends continue, the lake may soon stop freezing altogether, bringing an end to a centuries-old Shinto legend and the cultural heritage associated with it.

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